Engine exhaust gas heater

ABSTRACT

An improved engine exhaust gas heater based on the heat pipe principle is provided which is suitable for many applications, including aircraft to supply warm air for cabin conditioning, and also automobiles, trucks, and other vehicles. The heater is preferably associated with the engine exhaust gas muffler, and the resulting combination comprises a device or system of greater safety and of increased effectiveness regarding heat transfer and noise reduction capability.

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gas muffler, and ce or system of s regarding heat lllll ll [451 May 16, 1972 3,490,718 l/l970 3,493,177 2/1970 Bromberg.t..........................

FOREIGN PATENTS 0R APPLICATIONS 739,991 10/1943 Germany...............................

Primary ExaminerDouglas Hart Attorney-A. H. Caser ABSTRACT An improved engine exhaust gas heater based on the heat principle is provided which is suitable for man including aircraft to supply warm air for cabin conditi and also automobiles, trucks, and other vehicles. The heater preferably associated with the engine exhaust the resulting combination comprises a devi greater safety and of increased effectivenes transfer and noise reduction capability.

8 Claims, 7 Drawing Figures Alan J. Streb, Baltimore, Md.

Dynatherm Corporation, Cockeysville, Md.

Sept. 3, 1969 United States Patent Streb [54] ENGINE EXHAUST GAS HEATER [72] Inventor:

[73] Assignee:

[22] Filed:

[21] App]. No.:

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SHEET 1 HF 2 ALAN J. STREB m BY ATTORf-IEY PATENTEDMAY 16 I972 SHEET E OF 2 FIG. 2

FIG. 4

FIG. 6

ALAN J STREB A l TQM H ENGINE EXHAUST GAS HEATER CROSS-REFERENCES TO RELATED APPLICATIONS The heat pipe employed in FIGS. 4-7 is disclosed in copending application Ser. No. 854,827, filed Sept. 3, 1969 BACKGROUND OF THE INVENTION 1 The field of the invention comprises engine exhaust gas heaters.

2. So far as is known, the system described herein is new.

SUMMARY OF THE INVENTION The system employs a heat exchanger adjacent the engine exhaust gas muffler. Separate pathways are provided for the exhaust gas and the heating air or other heating medium, with the exhaust gas passing from the engine to the muffler to atmosphere, and the air circulating between the heat exchanger and the space to be heated. Thermal interconnection between the gaseous streams in the pathways is achieved by using one or more heat pipes, described below. Thus, physical failure, such as rupture, of either pathway will not result in commingling of the two streams, such as may occur in a conventional system and result in passage of toxic exhaust gases into the heated space or cabin.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated in the accompanying drawings, which are diagrammatic, and in which FIG. 1 is a side view of the device with parts broken away to show the interior construction;

FIG. 2 is a partial end view of the device of FIG. 1, looking in the direction of arrow A;

FIG. 3 is a view like FIG. 1, but reduced, and showing a modification;

FIG. 4 is a view like FIG. 3 but showing another modification in which a heat pipe of different construction is used;

FIG. 5 is an enlarged fragmental sectional view of FIG. 4 illustrating the heat pipe construction;

FIG. 6 is an enlarged fragmental sectional view along line 66 of FIG. 5; and

FIG. 7 is a view like FIG. 4, but showing another modification, and employing the heat pipe illustrated in FIGS. 5 and 6.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS Referring to FIG. 1, the heater system shown here is of particular applicability in aircraft, especially light aircraft whose cabin space is conventionally heated by means of the engine exhaust gases. The system includes a chamber 10, which in a light aircraft installation would comprise the muffler, to which engine exhaust gases are brought by the exhaust gas conduits, the latter termination in the baffles or perforated end pieces l1, 12 to which the conduits are attachable. The chamber is defined by a cylindrical wall 13 and end walls 14, 15 which are suitably apertured to receive the baffles. Exhaust gases leave by conduit 16.

A heat exchanger 17, preferably of cylindrical shape, is disposed adjacent the chamber as shown, and it will be noted that the baffle 11 is sufficiently elongated as to extend axially through the exchanger 17. Baffle 11 is supported at 18 by a concentric inner cylinder 19 which extends between, and is supported by, the apertured end walls 20, 21 of the heat exchanger. Cylinder 19 provides additional protection against contamination of the cabin heating air should the baffle 11, or its extension 11a, undergo rupture. Air may be introduced to the exchanger by an inlet 22 and leave by an outlet 23, for which suitable apertures are formed in the cylindrical wall 24.

A heat pipe, generally indicated at 25, extends between the chamber and the heat exchanger and provides a thermal interconnection therefor. It comprises a heat input section 26 disposed in the chamber 10, a heat output section 27 disposed in the heat exchanger 17, and a heat transfer member or element 28 which extends through and connects both sections.

Member 28 preferably is in the form of a continuous tube which extends from one end of the heat pipe to the other, while the sections 26 and 27 are characterized by the presence of closely spaced fins 29, 30 at each end portion of the tube. Heat input section 26 is heated by the hot exhaust gases brought into the chamber 10 by the exhaust gas conduits and the bafiles ll, 12, and the heat thus picked up is transported substantially isothermally by member 28 to the heat output section 27 where it is transferred by direct heat exchange to air flowing over fins 29 on its way to the space to be heated.

Although heat pipes per se are known, a brief description of them may be appropriate. Generally, each comprises a closed outer shell, usually in the form of a long tube such as the member 28, although other cross-sections are useful, in which a capillary structure or porous wick (not shown in FIGS. 1 and 2) is tightly and uniformly held against the inner surface of the shell throughout its length, and a working fluid or heat transfer agent is present in an amount sufficient to wet the entire wick. The shell may be formed of a variety of materials, either metallic or non-metallic and including ceramics and glass.

In operation, heat is added to the heat input section of the heat pipe, causing the fluid to evaporate, and thus the fluid gains heat by virtue of its latent heat of evaporation; fluid vapors then move through the pipe to the heat output section where, since this section is at a slightly lower temperature than the input section, they condense, giving up latent heat of condensation. The condensed fluid returns to the heat input section through the capillary action of the wick, and the foregoing cycle is repeated. Since the evaporation and condensation processes are isothermal, the transport of heat from one section of the heat pipe to the other takes place substantially isothermally, i.e., at a substantially uniform temperature.

While the capillary structure, also termed a porous wick, may have any suitable form, it should have pores of a sufflciently small size as to produce a capillary action. Generally, the capillary structure is an element separate from the tube, but it may also be secured to the same, or made integral therewith. The structure may comprise woven cloth, fiber glass, porous metal, porous ceramic structures such as tubes, wire screens, perforated metal sheets, and the like.

The working fluid of heat transfer agent includes practically any organic or inorganic liquid, or a molten salt, or a molten metal; some examples are hydrocarbons, fluorinated hydrocarbons, ketones like acetone, alcohols like methanol and glycerin, water, molten sodium chloride, molten elements like sodium, cesium, potassium, lead, bismuth, etc. The agent should not be corrosive nor should it decompose at operating temperatures. In the present case, an agent is chosen which can be vaporized in the range of temperatures prevailing in chamber 10. As these temperatures range from 800 to 1,800 F the agent may be chosen from such materials as sodium, potassium, cesium, and the like, which evaporate at tempera tures in the foregoing range.

Returning to FIG. 1, it will be seen that the heat pipe 25 is supported on the end wall 20 of heat exchanger 17 by a bolt and washer arrangement, one of which is indicated at 35; this comprises a plug 35a suitably secured in the recessed end 35b of the heat pipe, as by a weld, and having a threaded recess 35c into which the bolt 35d extends, the latter being provided with one or more washers 35e. The heat pipe is also supported on the end wall 21 by means of a pair of abutting plates 36, 37; plate 36, which is secured to member 28 as by welding, is disposed in an opening in wall 21, whereas plate 37, which is of larger diameter that plate 36, abuts the outer side of wall 21 and also abuts plate 36. A similar but reverse arrangement, involving plates 38, 39, is used to support the heat pipe on end wall 14 of chamber 10. A spacer sleeve 40 supports the plates 37 and 38. The entire heat pipe assembly may be stabilized in place by tightening the bolts as at 35.

It will be noted that the heat pipe mechanically and thermally connects the heat exchanger to the chamber. The elongated bafile l11la also acts to mechanically connect these structures, being in physical contact with the end wall 21 and also, at the stepped-up portion 41, being secured by means not shown, such as seam welding, to end wall 14.

As may be apparent from FIG. 2, a plurality of heat pipes are used, the configuration in FIG. 2 indicating the use of three heat pipes, two of which are indicated at 25 and 25a. A greater or lesser number may be used, as will be understood, and as is apparent, they are radially disposed about the baffles 11, 12. By varying the number of heat pipes, one can vary the amount of heat recovered from the exhaust gases, more heat pipes producing more heated air and vice versa. Of course, other conventional heat control means may be used, such as a step of mixing the heated air with air of different temperature. In general, the temperature of the heat pipe is a function of the conditions of the exhaust gas, i.e., temperature and flow rate, and of the cabin air, and will vary substantially over the range of operation.

By comparison with a conventional heating system, the present system enables more of the exhaust gas energy to be made available for heating the cabin space simply because a greater heat transfer surface area can be provided in the chamber 10.

As FIG. 1 shows, the muffler-heat exchanger combination is not significantly larger than the original muffier, considering the latter to be as represented by the chamber 10. The over all shape or envelope is substantially the same, enabling the new system to replace the conventional without appreciable difficulty. If required, other shapes or envelopes are possible simply by relocating the position of the heat exchanger; for example, the latter may be disposed angularly of the longitudinal axis of the muffler, thus maintaining the length and diameter of the original muffler while locating the heat exchanger in a selected position outwardly of the cylindrical surface of the muffler. Or by altering the construction of the heat pipe in a manner to be described, a simplification of the system of FIG. I may be achieved. Or by using the last-mentioned heat pipe of altered construction, the heat exchanger may be placed in a position outwardly concentric of the muffler so as to maintain the length of the original muffler while only moderately increasing its diameter. These variations are illustrated in FIGS. 3-7, which may now be described.

In FIG. 3, the heat exchanger 50 is disposed angularly of the longitudinal axis 51 of muffler 52, and such disposition may be at any suitable angle with respect to said axis. Hot exhaust gases enter the muffler via inlets 53 and 54, flows over heat input sections 55, 56, and 57 of heat pipes 58, 59, and 60, and leave the muffler via exit 61. Air on its way to the space to be heated enters heat exchanger 50 via inlet 62, is heated by heat output sections 63, 64, and 65 of the heat pipes, and leaves by outlet 66.

In FIG. 4-6, the heat exchanger 70 and muffier 71 are disposed end to end, as in FIG. 1, but the exhaust gas outlet in the muffler (note 16 of FIG. 1) is eliminated, thus providing a muffler without projections and improving the ease of installation of the system. The heat pipe 72 is constructed differently from those of FIGS. 1-3 but its principle is the same. It comprises a pair of concentric hollow structures, preferably the pipes or tubes 73, 74 (note FIG. which is an enlarged view, in section, of the upper right hand corner of the heat pipe 72 of FIG. 4), having therebetween a sealed evacuated annular space 75 which is disposed a capillary structure or porous wick 76 which is in contact with adjacent opposed surfaces of said pipes. Ends of the annular space 75, in which is disposed said wick, are sealed by rings, one of which is shown at 77 of FIG. 5. Suitably the capillary structure comprises, note FIGS. 5 and 6, a porous layer 76a in contact with outer surfaces of the inner of said pipes, another porous layer 76b in contact with inner surfaces of the outer of said pipes, and porous means 760 connecting said layers, and the entire structure may be made of fine mesh screen. The means 76c may be of any suitable shape and construction; in the form shown it is of cylindrical shape and is in contact with the opposed screen layers 76a and 76b. ltwill be understood that a plurality of cylinders 76c are circumferentially disposed in the annular space 75, and they may extend for the length of heat pipe 72, or they may be of shorter length and may be staggered circumferentially of the annular space. Layers 76a and 76b, and cylinder 76c, may be secured in place in any suitable way.

The capillary structure 76 may have other suitable forms, but in any event it should contain pores of a sufficiently small size as to produce a capillary action.

A heat transfer agent is represented at 78; it is one of the type described and substantially fills the capillary structure 76.

As hot exhaust gases enter the system at 80, they heat the inner tube at 74, which comprises the heat input section of the heat pipe, and the absorbed heat serves to vaporize the agent 78. The vapors pass toward the outer tube 73, which is at slightly lower temperature, where they condense, giving up latent heat of condensation. The condensed vapors are brought back toward the inner tube by virtue of the capillary action effect exerted by the capillary structure or wick 76, and they are re-vaporized, the foregoing process being repeated over and over. Air entering the heat exchanger 81 flows in the annular space or passage between the exchanger and heat pipe and is heated by the uniformly heated heat output section 73 of the heat pipe and leaves by outlet 82. Exhaust gases leave the system via exit 83. It will be seen that the heat transfer member or means of the heat pipe 72 comprises the wick and working agent disposed in the annular space 75.

A heat pipe like 72 is described at greater length in said copending application Ser. No. 854,827 filed Sept. 3, 1969.

The modification of FIGS. 4-6, in which the engine gas flows lineally through the system, is of particular interest for application in an automobile, truck, tractor, or other land vehicle.

In FIG. 7, the heat exchanger 85 is disposed in a position outwardly concentric of the muffler 86. Throughout most of its length the muffler is a part of the heat pipe 87, with only a portion 99 of the muffler extending beyond the heat pipe. The latter is constructed as in FIGS. 4-6 and need not be further described. Hot exhaust gases enter the system at 89 and 90 and leave by 91, and during their passage they heat up the heat input section of the heat pipe. Heat is transferred in the manner described in FIGS. 4-6, thus uniformly heating the heat output section, which in turn heats the air entering the heat exchanger via inlet 92. The air leaves by exit 93 on its way to the space to be heated.

In FIGS. 4-6 and 7, it will be noted that the heat pipe is hollow and forms a a part of the flow path of the exhaust gases. Also, in each case the heat exchanger is disposed outwardly concentrically of the heat pipe.

As may be more or less apparent, the described heating system is of use in any vehicle or installation employing an engine which produces hot exhaust gases and in which there is a space to be heated, including aircraft, automobiles, trucks, tractors, motor cycles, motor bikes, scooters, motor boats, and the like. In vehicles employing a muffler, the internal construction of the muffler may be variable, as to provide a more or less tortuous passageway and thus a greater noise level reduction, but even so, the invention is applicable therein. The heating system may also be of use in stationary installations, as illustrated by electric power-producing stations where an engine is used to operate a generator.

As indicated by the various views, the geometry of the heating system, by which is meant its size, shape, and disposition of parts, is variable to suit a given application.

The invention may be of further use in internal combustion engines to more effectively preheat the fuel gas mixture in the intake manifold on its way to the firing cylinders. Conventionally, such preheating is done at a hot spot, comprising a small chamber surrounding the mid-point of the intake manifold through which chamber a part of the hot exhaust gases is by-passed. According to the invention, one or more heat pipes can be used to thermally interconnect the hot exhaust gases in the exhaust gas manifold with the cool fuel gas in the intake manifold. It may also be feasible to use warm engine coolant instead of the hot exhaust gases, and in such case the interconnection will involve the duct carrying such coolant. It will be noted that the gas stream to be warmed comprises an air-hydrocarbon mixture.

It will be understood that the invention is capable of obvious variations without departing from its scope.

In the light of the foregoing description, the following is claimed.

I claim:

1. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprising a muffler to which exhaust gases from an engine are brought for gradual expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said muffler and disposed in endto-end relation thereto, a heat pipe connecting the muffler and heat exchanger and comprising (1) a heat input section disposed in said muffler and heated by hot exhaust gases, (2) a heat output section disposed in said heat exchanger, and( 3) a heat transfer member extending through opposed ends of said exchanger and muffler and connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, said heat pipe mechanically as well as thermally connecting said exchanger and muffler, means for introducing hot exhaust gases to said muffler, means for introducing air to said heat exchanger to be warmed by said heat output section, means for passing said warmed air to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler directly to atmosphere, said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases, and said muffler-heat exchanger-heat pipe system being not significantly larger than, and having substantially the same over all shape and diameter as, the muffler alone.

2. System of claim 1 wherein said exhaust gas introducing means comprises a pair of conduits which enter the muffler through opposed ends thereof with one of said conduits extending through the heat exchanger, and wherein means are present for supporting said last-mentioned conduit in the heat exchanger and for providing additional protection against contamination of the air by exhaust gases.

3. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprising a muffler to which exhaust gases from an engine are brought for gradual expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said muffler, a heat pipe connecting the muffler and heat exchanger and comprising (1) a heat input section disposed in said muffler and heated by hot exhaust gases, (2) a heat output section disposed in said heat exchanger, and (3) a heat transfer member connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, said heat pipe mechanically as well s thermally connecting said exchanger and muffler, means for introducing hot exhaust gases from the engine to said muffler, means for introducing air to said heat exchanger to be warmed by said heat output section, means for passing said warmed air to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler directly to atmosphere, and said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases.

4. System of claim 3 wherein said exhaust gas introducing means comprises a pair of conduits which enter the mufl'ler through opposed ends thereof, and wherein said system has substantially the same diameter and length as said mufiler alone.

5. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprisin a muffler to which exhaust gases from an engine are broug t for expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said mutiler, a heat pipe connecting the muffler and heat exchanger and comprising (i) a heat input section adapted to be heated by hot exhaust gases, 2) a heat output section for heating air, and (3) heat transfer means connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, means for introducing hot exhaust gases from said engine to said heat input section, means for introducing air to be warmed by said heat output section, means for passing said warmed air to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler to atmosphere, and said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases.

6. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprising a muffler to which exhaust gases from an engine are brought for gradual expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said muffler, a heat pipe connecting the muffler and heat exchanger and comprising l) a heat input section adapted to be heated by hot exhaust gases, (2) a heat output section for heating air, and (3) heat transfer means connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, said heat pipe being disposed entirely within the heat exchanger and extending longitudinally therein so as to form an annular passage therebetween for the flow of air, said heat pipe having along the longitudinal axis thereof a passageway which extends therethrough for the flow of hot exhaust gases, means for introducing hot exhaust gases from said engine to said passageway in the heat pipe, means for introducing air to said annular passage in said heat exchanger to be warmed by said heat output section, means for passing said warmed air from the annular passage to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler to atmosphere, and said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases.

7. System of claim 6 wherein said heat exchanger and muffler are disposed in end-to-end relation, wherein said heat pipe extends from end to end of the heat exchanger, and wherein said muffler-heat exchanger-heat pipe system has substantially the same diameter as the muffler.

8. System of claim 6 wherein said heat exchanger encloses a part of the muffler, wherein said muffler throughout a portion of its length is a part of the heat pipe, and wherein said muffler-heat exchanger-heat pipe system has substantially the same diameter as the mufiler. 

1. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprising a muffler to which exhaust gases from an engine are brought for gradual expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said muffler and disposed in end-to-end relation thereto, a heat pipe connecting the muffler and heat exchanger and comprising (1) a heat input section disposed in said muffler and heated by hot exhaust gases, (2) a heat output section disposed in said heat exchanger, and (3) a heat transfer member extending through opposed ends of said exchanger and muffler and connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, said heat pipe mechanically as well as thermally connecting said exchanger and muffler, means for introducing hot exhaust gases to said muffler, means for introducing air to said heat exchanger to be warmed by said heat output section, means for passing said warmed air to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler directly to atmosphere, said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases, and said muffler-heat exchanger-heat pipe system being not significantly larger than, and having substantially the same over all shape and diameter as, the muffler alone.
 2. System of claim 1 wherein said exhaust gas introducing means comprises a pair of conduits which enter the muffler through opposed ends thereof with one of said conduits extending through the heat exchanger, and wherein means are present for supporting said last-mentioned conduit in the heat exchanger and for providing additional protection against contamination of the air by exhaust gases.
 3. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprising a muffler to which exhaust gases from an engine are brought for gradual expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said muffler, a heat pipe connecting the muffler and heat exchanger and comprising (1) a heat input section disposed in said muffler and heated by hot exhaust gases, (2) a heat output section disposed in said heat exchanger, and (3) a heat transfer member connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, said heat pipe mechanically as well s thermally connecting said exchanger and muffler, means for introducing hot exhaust gases from the engine to said muffler, means for introducing air to said heat exchanger to be warmed by said heat output section, means for passing said warmed air to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler directly to atmosphere, and said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases.
 4. System of claim 3 wherein said exhaust gas introducing means comprises a pair of conduits which enter the muffler through opposed ends thereof, and wherein said system has substantially the same diameter and length as said muffler alone.
 5. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprising a muffler to which exhaust gases from an engine are brought for expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said muffler, a heat pipe connecting the muffler and heat exchanger and comprising (1) a heat input section adapted to be heated by hot exhaust gases, (2) a heat output section for heating air, and (3) heat transfer means connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, means for introducing hot exhaust gases from said engine to said heat input section, means for introducing air to be warmed by said heat output section, means for passing said warmed air to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler to atmosphere, and said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases.
 6. An engine exhaust gas heating system of improved safety and heat transfer efficiency comprising a muffler to which exhaust gases from an engine are brought for gradual expansion therein in order to reduce exhaust gas noise, a heat exchanger adjacent and separate from said muffler, a heat pipe connecting the muffler and heat exchanger and comprising (1) a heat input section adapted to be heated by hot exhaust gases, (2) a heat output section for heating air, and (3) heat transfer means connecting said input and output sections and serving to transport heat substantially isothermally from the input to the output sections, said heat pipe being disposed entirely within the heat exchanger and extending longitudinally therein so as to form an annular passage therebetween for the flow of air, said heat pipe having along tHe longitudinal axis thereof a passageway which extends therethrough for the flow of hot exhaust gases, means for introducing hot exhaust gases from said engine to said passageway in the heat pipe, means for introducing air to said annular passage in said heat exchanger to be warmed by said heat output section, means for passing said warmed air from the annular passage to a space to be heated, said air circulating between the exchanger and the space to be heated, means for discharging exhaust gases from said muffler to atmosphere, and said exhaust gases following a pathway from engine to discharge which is separate from the pathway followed by the air so that rupture of a wall of said muffler does not present a danger of contaminating said circulating air with said exhaust gases.
 7. System of claim 6 wherein said heat exchanger and muffler are disposed in end-to-end relation, wherein said heat pipe extends from end to end of the heat exchanger, and wherein said muffler-heat exchanger-heat pipe system has substantially the same diameter as the muffler.
 8. System of claim 6 wherein said heat exchanger encloses a part of the muffler, wherein said muffler throughout a portion of its length is a part of the heat pipe, and wherein said muffler-heat exchanger-heat pipe system has substantially the same diameter as the muffler. 